Water Quality at “The Lake”
A review of scientific articles published about Lake of the Ozarks
The University of Missouri Limnology Laboratory’s long-term monitoring on Lake of the Ozarks (LOTO), begun in 1976, resulted in three journal articles, all published in the proceedings of the International Society of Limnology. The first paper, published in 1981, described water quality in LOTO prior to the impoundment of Harry S. Truman Reservoir. In 1988 a second study evaluated changes in water quality associated with the completion of Truman Reservoir. A final paper published in 2000 examined seasonal water quality patterns. A brief summary of these three articles follows.
Limnological characteristics of Lake of the Ozarks, Missouri. 1981. J.R. Jones and J.T. Novak.
The purpose of this 1976–1979 study of summer water quality in LOTO was to establish baseline data prior to the impoundment of Truman Reservoir. Three main channel sites, were monitored, one each in the Gravois Mills, Grand Glaize and Niangua arms. Major findings included identification of a strong longitudinal gradient in water quality within the main lake channel. Average phosphorus concentrations at the 59-mile marker were three times higher than values at the dam (92 vs. 31 µg/L), with the 39-mile marker site having intermediate levels of 56 µg/L. Secchi transparency values were lowest up-lake (averaging 0.5 meters), moderate at the mid-lake site (1.2 meters) and deepest near the dam (2.0 meters). Interestingly, chlorophyll levels were similar across the three main lake sites, ranging from 14.2 µg/L at the up-lake site to 10.5 µg/L at the dam.
The chlorophyll concentrations at the 59-mile marker were low relative to phosphorus levels, averaging 0.15 unit of chlorophyll for each unit of phosphorus. In contrast, the ratio of chlorophyll to phosphorus at the dam site averaged 0.34. The inefficient use of phosphorus by algae at the up-lake site was attributed to high levels of inorganic suspended sediment, which reduced the penetration of sunlight into the lake and thus limited algal photosynthesis. While inorganic suspended sediment data were only measured in 1979, elevated turbidity readings and shallow Secchi transparency values at the up-lake site during 1976-1978 support the conclusion that suspended sediments reduced available light and limited algal growth.
Other findings included the relation between flow and lake water quality. Nutrient and turbidity levels were elevated and Secchi transparencies shallower in LOTO during years with high inflow. Also, data collected in the three tributary arms indicated water quality in the Gravois Mills and Grand Glaize arms was comparable to the dam site, while the Niangua Arm had slightly higher phosphorus values and shallower Secchi readings.
Limnological characteristics of Lake of the Ozarks, Missouri II: Measurements following formation of a large reservoir upstream. 1988. J.R. Jones and M.S. Kaiser.
The main objective of this study was to determine if water quality in LOTO changed after the completion of Truman Reservoir. The hypothesis was that Truman Reservoir would act as a settling basin, decreasing the amount of inorganic suspended sediment entering LOTO. Lower turbidity would allow more light to penetrate into the water column and lead to more algal growth, especially up-lake.
Data were collected during the summers of 1980-1986, with samples coming from the six previously monitored sites as well as two additional main lake sites (18- and 28-mile markers). Data showed that shifts in water quality were minimal and the strong longitudinal gradient in the main lake was still present during 1980-1986. Because inorganic suspended sediment measurements were limited to only one summer prior to the completion of Truman Dam, changes in suspended sediment concentrations could not be quantified.
Monitoring of surface water near the dam on Truman Reservoir revealed lower phosphorus and inorganic suspended sediment values than measured at the up-lake LOTO site. As expected, Truman Reservoir was acting as a settling basin with outflows containing moderate levels of nutrients and suspended sediment. It was theorized that releases from Truman Reservoir scour the Osage River bed, increasing the phosphorus and inorganic suspended sediment concentrations in water moving down-lake.
Chlorophyll concentrations at the up-lake site increased from 14.2 to 21.1 µg/L (average values for the two studies), even though phosphorus levels decreased from 92 to 78 µg/L. One possible explanation for the increase in chlorophyll relative to phosphorus (ratio increased to 0.27) is that a shift in the algal community might have occurred. Prior to the impounding of Truman Reservoir, the algal community in the up-lake portion of LOTO would have been dominated by species that were best suited for conditions in the Osage River. After 1979, the composition of the algal community in the upper portion of LOTO reflected that found in Truman Reservoir: species better suited for lake conditions.
Fewer sediments suspended in the water allowed light to penetrate deeper into the water column, which led to increased algal growth. Water clarity remained constant.
When water quality data were adjusted to account for differences in seasonal inflow, sites across the lake were determined to have higher chlorophyll concentrations (about 25% higher), even though phosphorus levels had dropped by about 26%. Secchi transparency values across the lake remained relatively unchanged after the completion of Truman Reservoir. Along with potential shifts in the algal community, lower inorganic suspended sediment concentrations in LOTO may account for increased algal production. In Missouri, both inorganic suspended sediment levels and algal biomass determine Secchi transparency. Because algal chlorophyll concentrations increased across the lake during 1980-1986 while Secchi measurements remained unchanged, it seems reasonable to surmise that inorganic suspended sediment levels decreased. In essence, the increased clarity associated with decreased inorganic suspended sediment was countered by increased algal biomass, which was made possible by the more favorable light environment that resulted from lower inorganic suspended sediment levels.
Limnological characteristics of Lake of the Ozarks, Missouri III: seasonal patterns in nutrients, chlorophyll and algal bioassays. 2000. B. D. Perkins and J.R. Jones.
Data collected throughout 1989-1991 and 1993 were examined to determine seasonal trends in water quality. While all sites were monitored, only data from two main lake channel sites (dam and 18 mile marker) were reviewed in the article.
Phosphorus, nitrogen, and inorganic suspended sediment displayed patterns of higher than average values April–June, decreasing concentrations through August, with increasing levels in the fall. The spring peak is associated with elevated inflows into the lake, while increases in fall were related to lake turnover. Decreasing nutrient and suspended sediment concentrations during summer occurred due to reduced inflow (generally) and the sedimentation of suspended materials (both inorganic and organic). Algal chlorophyll concentrations peaked in March and steadily decreased through spring and summer, with no fall peak.
Results from in-lake experiments determined that factors limiting algal growth in LOTO change over the course of the year. Limiting factors were phosphorus during spring, nitrogen during summer, and light in fall and early winter.
Past, Present and Future
One thing that is not in dispute about water quality in Lake of the Ozarks is its variability. These past studies, along with current LMVP monitoring, indicate that water quality (1) changes dramatically across the main channel, (2) varies among tributary arms, (3) differs from one year to the next, and (4) fluctuates within individual seasons. The variable nature of water quality in LOTO can be attributed not only to the vast size of the lake, but also to the influence of its immense watershed. Inputs of nutrients and suspended sediment differ as nonpoint and point sources of pollution vary across the landscape, with rainfall intensity and timing determining the impact. Differences in the timing and quantity of flow from Truman Reservoir also add to the variable nature of water quality in LOTO. Because the factors that influence water quality are many and their interactions complex, attempts to summarize water quality in LOTO require a large amount of data from numerous sites over several years. Continued monitoring by citizen volunteers will aid in evaluating differences in water quality across the lake, identifying trends in water quality over time, and assessing lake function (e.g. how water quality parameters relate to each other). Additional publications are planned to summarize this long-term data set.